Species interactions

1. Species interactions Effects on Interaction sp. 1 / sp. 2 • - / - Competition • - / 0 Amensalism • - / + Exploitation • 0 / + Commensalism • + / + Mutualism

2. Mutualism • Mutualism is not synonymous with symbiosis • Mutualism: mutually beneficial interaction • Symbiosis: living in intimate association • symbioses may be: • exploitative (e.g., Schistosomiasis, Heart worm) • mutualistic (e.g., bacteria in gut, zooxanthellae) • neutral (e.g., many bacteria on skin)

3. Mutualism vs. other interactions • Not as well-studied as predation & competition • Not modeled as frequently • Not studied experimentally as frequently

4. Facultative vs. Obligate • Obligate: Interaction is required for life of one or both members • Facultative: Interaction, though beneficial, is not absolutely required • For a pair of mutualists, the relationship may be facultative for one and obligate for the other

5. Examples of mutualisms • Flowering Plants & Pollinators • Bees, butterflies, flies, beetles, wasps, birds, bats, others • some obligate, some facultative • variable specificity

6. Examples of mutualisms • Cleaner mutualism • Fish, shrimp, some birds • Small organism removes ectoparasites from larger organisms • facultative, nonspecific

7. Examples of mutualisms • Ants & Aphids • Aphids feed on phloem • High sugar, low Nitrogen • Excess sugar excreted • Ants feed on sugar excretions • Ants defend aphids against predators • facultative for both

8. Examples of mutualisms • Mycorrhizal mutualism • Roots of most plants colonized by fungi • fungi provide H2O and nutrients, especially Phosphorus • plant provides carbohydrates

9. Ectomycorrhizal mutualisms • fungal mycelium covers root surface and penetrates between cells • most trees, forest mushrooms • for the fungus - facultative • for the plants - sometimes obligate

10. Endomycorrhizal (AM) mutualisms • ArbuscularMycorrhizal fungi • most non-woody plants • fungi penetrate plant cells • arbuscules: site of exchange • for the fungus - obligate • for the plant - facultative

11. Mutual altruism or mutual exploitation? • Mutualism has costsand benefits to both parties • Costs and benefits depend on the environment • cost > benefit … parasitism • cost < benefit … mutualism • Example: AM fungi and plants

12. root hairs ROOT fungal hyphae Zone of depletion (root hair) Zone of depletion (fungal hyphae) AM fungi and plant root • Benefit: Plant gets P • Roots deplete P in soil • Fungal hyphae extend plant’s ability to get P • Cost: Plant must give photosynthates to fungus • 7 to 20% of daily total

13. Low P soil Plants without mycorrhizal fungi cannot get enough P Mycorrhizal plants grow more rapidly than non-mycorrhizal plants Benefit > Cost Mutualism High P soil Plants obtain ample P even without mycorrhizal fungi Mycorrhizal plants grow less rapidly than non-mycorrhizal plants Benefit < Cost Parasitism Soil conditions alter relationship References: West 1997; Peng et al. 1993; Fitter 1985, 1991; Koide & Elliott 1989

14. High light High photosynthesis High carbohydrate availability Plant can easily afford to pay the cost of having AM fungi Mutualism Low light (shade) Low photosynthesis Low carbohydrate availability Plant less able to pay the cost of having AM fungi Parasitism Other potential effects on this relationship

15. General point • Mutualism depends on the balance of costs and benefits • For plants, that balance may vary in space and in time • Mutualism  Parasitism • For fungus, plant always provides net benefits (benefit > cost)

16. Direct vs. Indirect mutualism • Direct mutualism: Benefits occur between individuals, and without intermediaries • an individual phenomenon • may have population or community effects • Indirect mutualism: Benefits depend on effects on a 3rd species • often propagated through trophic network • a population phenomenon, usually with community effects

17. Dynamic effects of mutualism • How does mutualism affect population growth ( dN / dt )? • Increase equilibrium density • Increase maximum population growth • Increase both equilibrium density and maximum population growth • Increase neither equilibrium density and maximum population growth

18. Lotka-Volterra models of mutualism • Mutualism modeled as negative competition • Competition dN1 / dt = r1N1 [ K1 - N1 - a2N2] / K1 • a2 is the per capita effect of N2 reducing effective dN1/ dt • Mutualism dN1 / dt = r1N1 [ K1 - N1 + b2N2] / K1 • b2 is the per capita effect of N2 increasing effective dN1/ dt

19. N2 2-species equilibrium K2 K1 N1 Lotka-Volterra mutualism isoclines dN1/dt = dN2/dt = 0

20. Lotka-Volterra models of mutualism • Isoclines have positive slopes • Equilibrium exists and is stable if b1b2 < 1.0 • Both species attain greater equilibrium densities when together vs. when alone. • Equilibrium is undefined if b1b2 > 1.0 • Isoclines don’t cross • Both species increase to 

21. Lotka-Volterra models of mutualism • Suppose mutualism coefficients are not constants • Suppose b’s decrease as N1 and N2increase • At sufficiently high N‘s, mutualist has no effect on dN / dt • Stabilty • Increase equilibrium density

22. N2 2-species equilibrium K2 K1 N1 Modified Lotka-Volterra mutualism isoclines dN1/dt = dN2/dt = 0

23. r1 rmax N2 k1/2 Increased maximum dN / dt • Suppose r1 is an increasing function of N2 dN1 / dt = r1 N1[ K1 - N1 ] / K1 • where: r1 = rmax N2 / [ k1/2 +N 2] • as N2 increases, r1  rmax

24. Example: Pollinators • Suppose K for the plant is set by available space (or by soil nutrients or by light) • Pollinators may increase seed set and therefore increase maximum dN / dt • Pollinators don’t alter available space so don’t alter K

25. Inreased equilibrium density and maximum dN / dt • Suppose r1 is an increasing function of N2 dN1 / dt = f (N2) N1 [ K1 - N1 + b2N2] / K1 • where: f (N2)is an increasing function of N2 • Example: Ants & Aphids • ants increase survivorship & reproduction of aphids (increase dN / dt ) • ants alter equilibrium density (by local elimination of enemies)

26. No effect on equilibrium density and maximum dN / dt • Benefits of mutualism may be to individuals • Those that have mutualists have advantage in intraspecific competition • But, no necessary advantage at population level.

27. Resource based mutualisms • Lotka-Volterra models phenomenological • Resource based models mechanistic • Resource processing • Resources may pass through a series of transformations caused by their use by organisms RESOURCE MODIFIED RESOURCE

28. Resource processing • When resource is changed by processing, it may become valuable to another species • Different consumers may specialize on the same resource in different states • Processing chain • Models: • Heard, S. B. 1994. J. Anim. Ecol. 63:451-466 • Heard, S. B. 1995. Ecol. Modelling80:57-68 • Experiments • Daugherty & Juliano 2002. Ecological Entomology

29. Processing chain examples • Vinegar • yeast consume sugar, excrete alcohol • Acetobacterconsume alcohol, excrete acetic acid • Stream invertebrates • leaves, eaten by shredders (e.g., pteronarcid stoneflies, tipulid flies) • defecate fine particles, fed upon by filterers, gatherers (e.g., simuliids - blackflies, mayflies, chironomids)

30. Processing chain examples • Predation & scavenging • large predators (e.g., lions) • carrion eaters (e.g., vultures) • Fish carrion • Bald eagle • Crows, gulls • Many other examples (see Heard 1994, Journal of Animal Ecology 63:451-464)

31. supply p consumption f1(R1,S1) loss w1(R1) consumer dependent processing s [f1(R1,S1)] consumer independent processing h (R1) loss w2(R2) consumption f2(R2,S2) A two species processing chain Resource condition 1 (upstream) R1 Consumer 1 (upstream) Resource condition 2 (downstream) R2 Consumer 2 (downstream)

32. Processing has multiple effects • Upstream consumer exploits the resource • reduces its value (e.g., energy content) • Downstream consumer cannot use the resource until it is processed • by upstream consumer • by other means • Upstream consumer, by processing, makes resources available to downstream consumer

33. A simple processing chain model • R1… upstream resource • R2 … downstream resource • S1… upstream consumer • S2 … downstream consumer • p … supply rate of upstream resource • h(R1) … consumer independent processing • w1(R1) … loss rate for upstream resource • w2(R2) … loss rate for upstream resource

34. A simple model of a processing chain • f1 (R1 , S1) … resource consumed by upstream consumer • f2 (R2 , S2) … resource consumed by downstream consumer • note: feeding functions could be in the form of saturation kinetics models. • s … fraction of upstream resource that is used by the upstream consumer and made available to the downstream consumer

35. A simple model of a processing chain • m1 … upstream consumer per capita mortality • m2 … downstream consumer per capita mortality • g1 … growth per unit resource consumed for upstream consumer • g2… growth per unit resource consumed for downstream consumer

36. A simple model of a processing chain • dR1/ dt = p - h(R1) - w1(R1) - f1(R1,S1) • dR2/ dt = h(R1) - w2(R2) + s [f1(R1,S1)] - f2(R2,S2) • dS1/ dt = g1{(1 - s)[f1(R1,S1)]} - m1(S1) • dS2/ dt = g2{[f2(R2,S2)]} - m2(S2) • equilibrium • growth rates = 0 • S1* , S2* , R1* , R2*

37. Commensal vs. Amensal • Effects are one way • S1 can affect S2, but S2cannot affect S1 • Consider equilibrium S2when: • S1 > 0 … upstream consumer present • S1 = 0 … upstream consumer absent • S2* | S1 > 0 > S2*| S1 = 0 … commensal • S2* | S1 > 0 < S2*| S1 = 0 … amensal

38. What determines commensal vs. Amensal • Equilibrium commensalism occurs when s > h / (h + w1) • sloppiness (s) is large • loss of upstream resource (w1) is large • consumer independent processing (h) is small • However, allequilibrium amensal relationships can behave as commensal relationships prior to equilibrium • short term increases in S2 due to S1

39. Resource based mutualisms • Holland & DeAngelis 2010. Ecology 91:1286–1295 • Multiple resource-consumer mutualisms • facultative, obligate, one-way, two-way • e.g., mycorrhizae • plant provides fungus with carbohydrate • Fungus provides plant with P

40. Resource based mutualisms dM1/dt = M1[r1 + c1(Fmax12M2/(K2+M2)) – q1(Smax1M2/(e1+M1)) – d1M1] dM2/dt= M2[r2+ c2(Fmax21M1/(K1+M1)) – q2(Smax2M1/(e2+M2)) – d2M2] Mi = number or biomass Fmax, K = Feeding paramters Smax, e = supply paramters di = death rate ci , qi = conversion efficencies

42. Key results • Mutualisms can yield multiple equilibrium abundances • Some stable, some not. • Depends on • unidirectional/bidirectional • obligate/facultative • Overexploitation and extinction of a mutualist are possible • Transient dynamics may be inconsistent with the mutualism effect on equilibrium abundance

43. Consumer-Resource mutualisms • Takes a cost – benefit approach • But puts a mechanism on costs and benefits (consumption) • Places costs and benefits into a common currency

44. competitor #1 food competitor #2 space competitor #3 Indirect mutualism • Beneficial interactions depend on a 3rd species or a network • predation on predators, competitors • competitor of predators, competitors • positive effects necessarily at the population level

45. Indirect mutualism(Fritz 1983) • Insects on Black Locust • Herbivores • Vanduzea -- membracid treehopper, honeydew • Odontota -- chrysomelid beetle, leaf miner • other herbivores • Ant • Formica -- tends Vanduzea, feeds on honeydew, attacks other herbivores and predators • Predator • Nabicula -- nabid bug, preys on Odontota Chris Evans , The University of Georgia

46. Ant mutualist effects on plant? • Branches with ants have fewer other herbivores • Branches with ants have 51% fewer Odontotaadults • Ants benefit plant? • Exclude ants from small trees & compare to control • With ants: fewer Odontota, Nabicula • With ants: Odontota larvae survive at greater rate • With ants: Greaterdefoliation • Net effect of ants on plant is detrimental • depends on strengths of direct & indirect effects

47. Nabicula Formica Vanduzea Odontota Other herbivores Black Locust Black Locust system

48. Community level effects of mutualism • How might direct mutualisms affect community properties? • species number & relative abundance • Can mutualists reduce the impact of competition? Or predation/herbivory? • keystone mutualists • read: Morris et al. 2007 Ecology

49. Microcosm experiment(Grime et al. 1987) • Festuca ovina - canopy dominant • 20 other species • Treatments • Grazing (scissors) • Soil heterogeneity • AM fungi

50. Microcosm experiment(Grime et al. 1987) • Grazing • appeared to increase number of species slightly • appeared to increase diversity index • AM • appeared to increase number of species slightly • appeared to increase diversity index • NOTE - no real data analysis here